1. Field of the Invention
The present invention relates generally to mixers and more particularly to integrated-circuit mixers.
2. Description of the Related Art
Basically, a mixer multiplies two signals in a nonlinear process which generates intermodulation products whose frequencies are the sums and differences of integral multiples of the input signal frequencies. For example, if the input signals have angular frequencies of .omega..sub.1 and .omega..sub.2, the intermodulation products will have frequencies of n.omega..sub.11 .+-.m.omega..sub.2. In particular, sum and difference signals are generated which have frequencies of .omega..sub.1 +.omega..sub.2 and .omega..sub.1 -.omega..sub.2 respectively. Other exemplary intermodulation products are second-order products (harmonics) having frequencies such as 2.omega..sub.1 and 2.omega..sub.2 and third-order products having frequencies such as 2.omega..sub.1 -.omega..sub.2 and 2.omega..sub.2 -.omega..sub.1.
In an exemplary mixer use, an intermediate frequency (IF) signal is mixed with a local oscillator (LO) signal to generate (typically with filtering to remove undesired intermodulation products) a radio-frequency (RF) signal (i.e., the IF signal is upconverted by the LO signal to form an RF signal). In this use, the ratio of the RF signal to the IF signal is typically referred to as the mixer's conversion gain.
Conversely, an RF signal can be mixed with an LO signal to generate (again with filtering to remove undesired intermodulation products) an IF signal at a frequency where filtering and/or gain may be easier to implement (i.e., the RF signal is downconverted by the LO signal to form an IF signal). In this use, the ratio of the IF signal to the RF signal is typically referred to as the mixer's conversion gain.
When the fundamental frequencies .omega..sub.1 and .omega..sub.2 are close to one another, the third-order intermodulation products 2.omega..sub.1 -.omega..sub.2 and 2.omega..sub.2 -.omega..sub.1 lie in the region of the fundamentals and are difficult to filter out. Accordingly, the nonlinear elements of mixers are sometimes placed in balanced arrangements which oppose the third-order intermodulation products of the elements to thereby reduce their amplitude at the mixer output. The third-order intercept point is a mixer figure of merit. Increasing the third-order intercept point of a mixer reduces the amplitude of its third-order intermodulation products.
The nonlinearities of semiconductor diodes and transistors have been extensively employed in mixer structures. Integrated circuit mixers are often realized with transistors which not only produce the necessary nonlinear multiplication process but provide conversion gain as well.
FIG. 1 illustrates an exemplary mixer 20 which has an isolation amplifier 22 inserted between an input differential amplifier 24 and an output differential amplifier 26. The input differential amplifier 24 includes a pair of transistors 27 and 28 whose emitters are respectively coupled to ground through current sources 31 and 32 and are coupled together by a resistor 34. The bases of the transistors 27 and 28 form a first input port 36 and output paths of the input differential amplifier 24 are formed by the collectors 37 and 38.
The output amplifier 26 includes two differential pairs 40 and 42 of transistors 44. The bases of the differential pairs are cross-coupled to form a second input port 50. The collectors of the differential pairs are cross-coupled and connected to a bias port 52 by output resistors 53 and 54. The cross-coupled collectors also form an output port 56.
The emitters of the differential pairs 40 and 42 are connected to the output paths 37 and 38 of the transistors 30 and 32 by the isolation amplifier 22. In particular, the isolation amplifier has isolation transistors 60 and 62 which are respectively coupled in cascode relationships with transistors 27 and 28 of the differential input amplifier 22. That is, the emitters of isolation transistors 60 and 62 are respectively connected to the collectors 37 and 38 of the input transistors 27 and 28. Collectors of the isolation transistors 62 and 64 are respectively connected to the emitters of the differential pairs 40 and 42 and bases of the isolation transistors 62 and 64 are connected to a bias port 66.
The isolation amplifier 22 facilitates the introduction of trickle currents 68 and 69 by current resistors 70 and 71 which are coupled between the collectors 37 and 38 of the input differential amplifier and the bias port 52. The magnitudes of the trickle currents are set by a voltage at the bias port 66 and by the impedance of the current resistors 70 and 71.
The mixer 20 is of a type conventionally referred to as a "Gilbert mixer". In an exemplary operation of the mixer 20, a first signal having an angular frequency of .omega..sub.1 is applied at the first input port 36 and a second signal having an angular frequency of .omega..sub.2 is applied at the second input port 50. First-signal currents generated by the input differential amplifier 24 are passed through the isolation amplifier 22 to generate differential first-signal currents in the output differential amplifier 26. The second signal also generates differential currents in the output differential amplifier 26 which mix with the first-signal differential currents to produce intermodulation products at the output port 56.
More particularly, the transistors 44 of the output differential amplifier 24 are essentially arranged in a mixing ring. First-signal currents are applied across the cross-coupled emitters of this ring and second-signal currents are applied to the cross-coupled bases of the ring. Currents of the first signal are switched on and off at the rate of the second signal to generate intermodulation products which are coupled to the output port 56 from collectors at opposite sides of the ring.
The balanced arrangement of the mixing ring facilitates a reduction of the second-order intermodulation products. Introduction of the trickle currents 68 and 69 increases the collector currents of the input differential amplifier 24 above the currents 72 and 73 that flow through the isolation amplifier 22. The added current enhances the amplifier's transconductance and, thus, the mixer's conversion gain and its third-order intercept point. Unfortunately, the current resistors 68 and 69 provide paths from the bias port 52 for the introduction of power supply fluctuations that modulate the transconductance of the input differential amplifier 24. This effect can be reduced by increasing the magnitude of current resistors 70 and 71 but this reduces the trickle currents 68 and 69 and the advantages which they provide. In addition, resistors 53 and 54 provide paths which introduce power supply noise into the output port 56.